A construction of a transformer is standardized according to IEC (International Electrotechnical Communication) standard Pub. 60950 and the likes. That is, the standards define that at least three insulating layers be formed between primary and secondary windings (an enamel film which covers a conductor of a winding is not considered as an insulating layer), or that a thickness of an insulating layer be 0.4 mm or more. The standards also provide that a creepage distance between the primary and secondary windings, which varies depending on an applied voltage, be 5 mm or more, and that the transformer should withstand a voltage of 3,000 V, applied between the primary and secondary sides, for a minute or more, and the like.
According to the standards, as a currently available transformer, a construction illustrated in a cross-section view of FIG. 2 has been adopted. An enameled primary winding 4 is wound around a bobbin 2 on a ferrite core 1 in a manner such that insulating barriers 3 for securing a creepage distance are arranged individually on opposite sides of a peripheral surface of the bobbin 2. An insulating tape 5 is wound for at least three turns on the primary winding 4. The insulating barriers 3 for securing the creepage distance are further arranged on the insulating tape, and then an enameled secondary winding 6 is wound around the insulating tape.
However, in the recent years, a transformer having a structure that includes neither the insulating barrier 3 nor the insulating tape layer 5, as shown in FIG. 1, has been used instead of the transformer having the sectional structure shown in FIG. 2. The transformer shown in FIG. 1 has advantages in that an overall size thereof can be reduced compared to the transformer having the structure shown in FIG. 2, and that work for winding the insulating tape can be omitted.
In manufacturing the transformer shown in FIG. 1, it is necessary, in consideration of the above mentioned IEC standards, that at least three insulating layers 4b (6b), 4c (6c), and 4d (6d) are formed on the outer peripheral surface on one or both of conductors 4a (6a) of the primary winding 4 and the secondary winding 6.
As such a winding, there is known a structure in which an insulating tape is wound firstly around an outer circumference of a conductor to form a first insulating layer thereon, and is further wound to form second and third insulating layers in succession, so as to form three insulating layers that are separable from one another. In addition, there is known a winding structure in which a fluorine resin in place of the insulating tape is successively extrusion-coated on the outer circumference of the conductor to form three insulating layers in all.
In the above-mentioned case of winding the insulating tape, however, because winding the tape is an unavoidable operation, the efficiency of production is extremely low, and thus a cost of the electrical wire is conspicuously increased.
In addition, in the case of extruding the fluorine resin, there is an advantage in that the insulating layers have good heat resistance, because they are formed of the fluorine resin. However, there are problems in a high cost of the resin. Further, when the fluorine resin is pultruded at a high shearing speed, an external appearance thereof tends to be deteriorated. Accordingly, it is difficult to increase a production speed thereof, thereby increasing a cost of the electric wire as in the case of winding the insulating tape.
In attempts to solve such problems, a multilayer insulated electric wire is applied to a practical use. In the multilayer insulated electric wire, a modified polyester resin with controlled crystallization to suppress an decrease in a molecular weight thereof is extruded around a conductor to form first and second insulating layers, and a polyamide resin is extruded to form a third insulating layer. Further, with the recent trend in reducing a size of electrical/electronic devices, an effect of heat on the devices has been a concern. Accordingly, a multilayer insulated electric wire with improved heat resistance has been proposed, in which a polyether-sulfone resin is extruded and coated as an inner layer, and a polyamide resin is extruded and coated as an outermost layer.
When a transformer is attached to a device after coil winding to form a circuit, a conductor is exposed from a distal end of an electric wire drawn from the transformer, so that soldering is performed thereon. With further reduction in a size of electrical/electronic devices, there is a need to develop a multilayer insulated electric wire, in which coating layers are not cracked, even when a covered electric wire portion drawn from a transformer is subjected to soldering after processing such as bending, and in which the covered electric wire can be subjected to a processing such as bending properly.